This invention relates to negative electrodes for use in batteries (e.g., lithium electrodes for use in lithium-sulfur batteries). More particularly, this invention relates to alkali metal electrodes having a thin glassy or amorphous protective layer.
The rapid proliferation of portable electronic devices in the international marketplace has led to a corresponding increase in the demand for advanced secondary batteries (i.e., rechargeable batteries). The miniaturization of such devices as, for example, cellular phones, laptop computers, etc., has naturally fueled the desire for rechargeable batteries having high specific energies (light weight). At the same time, mounting concerns regarding the environmental impact of throwaway technologies, has caused a discernible shift away from primary batteries and towards rechargeable systems.
Among the factors leading to the successful development of high specific energy batteries, is the fundamental need for high cell voltage and low equivalent weight electrode materials. Electrode materials must also fulfill the basic electrochemical requirements of sufficient electronic and ionic conductivity, high reversibility of the oxidation/reduction reaction, as well as excellent thermal and chemical stability within the temperature range for a particular application. Importantly, the electrode materials must be reasonably inexpensive, widely available, non-explosive, non-toxic, and easy to process.
In theory, some alkali metals could provide very high energy density batteries. The low equivalent weight of lithium renders it particularly attractive as a battery electrode component. Lithium also provides greater energy per volume than does the traditional battery standards, nickel and cadmium. Unfortunately, no rechargeable lithium metal batteries have yet succeeded in the market place.
The failure of rechargeable lithium metal batteries is due in large measure to cell cycling problems. To be commercially viable, a lithium battery should recharge at least a hundred times. On repeated charge and discharge cycles, lithium "dendrites" gradually grow out from the lithium metal electrode, through the electrolyte, and ultimately contact the positive electrode. This causes an internal short circuit in the battery, rendering the battery unusable after a relatively few cycles. While cycling, lithium electrodes may also grow "mossy" deposits which can dislodge from the negative electrode and thereby reduce the battery's capacity.
To address some of the cycling problems observed with lithium metal electrodes, some researchers have developed lithium batteries employing a solid electrolyte, such as an ionically conductive polymer or ceramic. Note that most traditional batteries employ liquid electrolytes. It has been found that systems employing such solid electrolytes reduce the incidence of dendrites and mossy deposits. Unfortunately, solid electrolytes also possess a relatively low ionic conductivity (in comparison to liquid electrolytes), thereby reducing the high rate discharge (high power) performance of the battery.
To address lithium's poor cycling behavior in liquid electrolyte systems, some researchers have proposed that the electrolyte facing side of the lithium negative electrode be coated with a "protective layer." Such protective layer must conduct lithium ions, but at the same time prevent contact between the lithium electrode surface and the bulk electrolyte. Known protective layers all have certain difficulties.
Many lithium metal protective layers contemplated to date form in situ by reaction between lithium metal and compounds in the cell's electrolyte which contact the lithium. Most of these in situ films are grown by a controlled chemical reaction after the battery is assembled. Generally, such films are of poor quality, having a porous morphology allowing some electrolyte to penetrate to the bare lithium metal surface.
Some research has focused on "nitridation" of the lithium metal surface as a means for protecting lithium electrodes. In such process, a bare lithium metal electrode surface is reacted with a nitrogen plasma to form a surface layer of polycrystalline lithium nitride (Li.sub.3 N). This nitride layer conducts lithium ions and at least partially protects the bulk lithium of the negative electrode from a liquid electrolyte. A process for nitriding lithium battery electrodes it is described in R&D Magazine, September 1997, p 65 (describing the work of S. A. Anders, M. Dickinson, and M. Rubin at Lawrence Berkeley National Laboratory). Unfortunately, lithium nitride layers suffer from various problems. First, the grain boundaries between crystallites in a lithium nitride layer offer pathways for electrolyte to find its way to the bulk lithium electrode. In addition, lithium nitride decomposes when exposed to moisture. While lithium metal batteries employ nonaqueous electrolytes, it is very difficult to remove all traces of moisture from the electrolyte. Thus, trace moisture will ultimately compromise the protective properties of the lithium nitride. Still further, lithium nitride has a very low voltage window. That is, it is stable over only very limited potential differences. Specifically, when exposed to a potential difference of greater than about 0.45V, it oxidizes. This makes it unsuitable for applications where it may be exposed to higher potential differences, as when the lithium electrode becomes highly polarized.
Other pre-formed lithium protective layers have been contemplated. Most notably, U.S. Pat. No. 5,314,765 (issued to Bates on May 24, 1994) describes a lithium electrode containing a thin layer of sputtered lithium phosphorus oxynitride ("LiPON") or related material. LiPON is a glassy single ion (lithium ion) conductor which has been studied as a potential electrolyte for solid state lithium microbatteries that are fabricated on silicon and used to power integrated circuits (See U.S. Pat. Nos. 5,597,660, 5,567,210, 5,338,625, and 5,512,147, all issued to Bates et al.). Unfortunately, sputtering is a cold process and so the sputtered LiPON layer may frequently be porous and have columnar structures, limiting its usefulness in protecting lithium.
Lithium battery technology still lacks an effective mechanism for protecting lithium negative electrodes from degradation during extended cell cycling. Thus, before lithium metal batteries become commercially viable, such protective mechanism must be developed.